Optical deflector

ABSTRACT

An optical deflector comprises a control means for controlling at least either a light source or a deflection means adapted to gauge the distance between the position of a deflected beam of light moving on a light receiving element in one direction and the position of another deflected beam of light moving on the light receiving element in the opposite direction and control the distance so as to make it agree with a predetermined value. Thus, the optical deflector can very accurately control the operation of the deflection means in such a way that it is not affected by changes of environmental temperature of the deflection means and the detection circuit, because a detection means for detecting the time when a beam of light passes by a predetermined angle of deflection of the deflection means is not used.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to an optical deflector having adeflection means for deflecting light.

[0003] 2. Related Background Art

[0004]FIG. 1 of the accompanying drawings illustrates a galvano-mirroras an example of optical deflector that is driven by electromagneticforce. A mirror is arranged on a movable section, which is supported bya main body by way of a pair of torsion bars so that it may be rotatedrelative to a central axis. In FIG. 1, reference symbol 50 denotes asilicon substrate and reference symbols 51 and 52 respectively denote anupper glass substrate and a lower glass substrate. There are also showna movable plate 53, a pair of torsion bars 54, a planar coil 55, a totalreflection mirror 56, a pair of electrode terminals 57 and permanentmagnets 60 through 63. The illustrated optical deflector is of theelectromagnetic type that is driven by causing a drive current to flowthrough the planar coil 55 and utilizing the Lorentz force that isgenerated by the drive current and the permanent magnets (see, interalia, U.S. Pat. No. 5,606,447).

[0005] Japanese Patent Application Laid-Open No. 2001-305471 describesan electromagnetic actuator. This patent document has much in commonwith U.S. Pat. No. 5,606,447 in that a movable part is driven byelectromagnetic force. The electromagnetic actuator disclosed inJapanese Patent Application Laid-Open No. 2001-305471 also has a totalreflection mirror arranged on a movable part.

[0006] Japanese Patent Application Laid-Open No. 2001-305471 describesas follows in terms of problems, objects and means. The inventiondisclosed in the above patent document paid attention to the fact thatthe resonance period of an electromagnetic actuator normally drifts withtemperature and time and hence, if an electric current having apredetermined, constant resonance frequency is continuously supplied tothe planar coil, there arises a problem that it is not possible tocontrol the angle of deflection and keep it to a constant value withtemperature change and time lapse. Thus, the first object of thatinvention is to provide an electromagnetic actuator that can be drivento reciprocate with its resonance period without providing a separatedetection means as well as a drive control device and a drive controlmethod to be used for such an electromagnetic actuator and the secondobject of that invention is to provide an electromagnetic actuator whoseangle of deflection can be controlled without providing a separatedetection means as well as a drive control device and a drive controlmethod to be used for such an electromagnetic actuator, while the thirdobject of the invention is to provide a resonance frequency signalgenerating device and a resonance frequency signal generating method tobe used for an electromagnetic actuator that can output a resonancefrequency signal corresponding to the resonance period of theelectromagnetic actuator.

[0007] The invention of the above cited patent document utilizes a coilas means for exciting the movable section of the electromagneticactuator and also as detection means. The induced voltage or current inthe coil is utilized for detection.

[0008] While Japanese Patent Application Laid-Open No. 2001-305471describes that the resonance period of an electromagnetic actuatordrifts with temperature and time, it proposes to detect the time whenthe actuator passes by a predetermined angle of revolution (deflection)from the coil that is a detection means as time-related information.

[0009] U.S. Pat. No. 5,606,447 does not pay attention to the problemthat the resonance period of an electromagnetic actuator drifts withtemperature.

[0010] With the method of detecting the time when the actuator passes bya predetermined angle of deflection gives rise to a signal delay in thedetection circuit due to changes of environmental temperature.Additionally, timing errors can occur when gauging the change with timeof the angle of deflection and detecting the time on the basis of thegauged change because the detection timing can be shifted by signalnoises and offsets.

[0011] Thus, the problem of signal delays and timing errors arises whenaccurately controlling an actuator by such a method of detecting thetime when the actuator passes by a predetermined angle of deflection.

SUMMARY OF THE INVENTION

[0012] The inventor of the present invention came up with an ideadifferent from that of the inventor of the invention disclosed inJapanese Patent Application Laid-Open No. 2001-305471. Morespecifically, it is the object of the present invention to make itpossible to very accurately control the operation of a deflection means,or an actuator, without using a detection means for detecting the timewhen a beam of light passes by a predetermined angle of deflection, oractuator, in such a way that it is not affected by changes ofenvironmental temperature of the deflection means and the detectioncircuit.

[0013] Thus, according to the invention, there is provided an opticaldeflector having a deflection means for deflecting modulated light froma light source so as to make deflected beams of light scan, said opticaldeflector comprising a control means for gauging a distance between aposition of a deflected beam of light moving on a light receivingelement in one direction and a position of another deflected beam oflight moving on the light receiving element in the opposite directionand controlling at least either the light source or the deflection meansso as to make the distance agree with a predetermined value.

[0014] In another aspect of the invention, there is provided a method ofcontrolling an optical deflector adapted to deflect light from a lightsource so as to make deflected beams of light scan, the methodcomprising: a position detecting step of detecting a position of adeflected beam of light moving on a light receiving element in onedirection and a position of another deflected beam of light moving onthe light receiving element in the opposite direction; a step of sensinga phase difference between a phase as detected in the detecting step anda predefined phase; and a step of controlling either a drive frequencybeing applied to the optical deflector or a modulation timing forreciprocative drawing according to an outcome of the sensing step.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic illustration of an optical deflector;

[0016]FIG. 2 is a schematic illustration of scanning of beams of lightthat are deflected (reflected) by the deflection means of the firstembodiment of optical deflector so as to reciprocate on a lightreceiving element, showing the deflected beams of light scanning only inone direction along with the trajectory thereof;

[0017]FIG. 3 is a schematic cross sectional view of the first embodimentof optical deflector taken along a plane containing a beam of lightdeflected by the deflection means;

[0018]FIGS. 4A, 4B and 4C are graphs illustrating exemplary drivewaveforms that can be applied to the deflection means 202 of the firstembodiment of optical deflector;

[0019]FIG. 5 is a schematic block diagram of the first embodiment ofoptical deflector, illustrating the control flows thereof;

[0020]FIGS. 6A, 6B and 6C are schematic illustrations of the secondembodiment;

[0021]FIG. 7 is a schematic illustration of a plurality of lightreceiving elements that the light receiving element of the thirdembodiment includes;

[0022]FIG. 8 is a schematic cross sectional view of the fourthembodiment of optical deflector taken along a plane containing a beam oflight deflected by the deflection means;

[0023]FIGS. 9A and 9B are graphs illustrating the frequencycharacteristics of the resonance type deflector of the fifth embodimentof optical deflector;

[0024]FIGS. 10A and 10B are graphs illustrating drive signal 309 of thedeflection means 202 of the resonance type deflector of the fifthembodiment of optical deflector and the change with time of the angle ofdeflection at the time when the drive signal 309 is applied;

[0025]FIGS. 11A and 11B are schematic illustrations of a method ofgenerating modulated spots by the sixth embodiment of optical deflector;

[0026]FIGS. 12A and 12B are schematic illustration of another method ofgenerating modulated spots by the sixth embodiment of optical deflector;

[0027]FIG. 13 is a schematic illustration of the seventh embodiment ofoptical deflector;

[0028]FIG. 14 is a schematic illustration of the light receiving element101 arranged within a scanning area 214 and the display region of theseventh embodiment of optical deflector;

[0029]FIG. 15 is a schematic illustration of the eighth embodiment ofoptical deflector;

[0030]FIG. 16 is a schematic illustration of the configuration of thedevice of Example 1;

[0031]FIG. 17 is a flow chart of the operation of Example 1;

[0032]FIG. 18 is a flow chart of the operation of Example 2;

[0033]FIGS. 19A and 19B are schematic illustrations of the configurationof the apparatus of Example 3; and

[0034]FIG. 20 is a flow chart of the operation of Example 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] (First Embodiment)

[0036] The inventor of the present invention came to have an idea ofutilizing deflected beams of light that are emitted from a light sourceand deflected to reciprocate for scanning by a deflection means thatalso reciprocates (swings), for controlling at least either the lightsource or the deflection means.

[0037] More specifically, according to the invention, deflected beams oflight moving forward and moving backward are detected by a lightreceiving element and either the deflection means or the light source iscontrolled by way of a control means in such a way that the distance(displacement) between the position of the forwardly moving deflectedbeam of light and that of the backwardly moving deflected beam of light,each at a given clock time, shows a predetermined value.

[0038]FIG. 2 is a schematic illustration of scanning of beams of lightthat are deflected (reflected) by the deflection means of the firstembodiment of optical deflector so as to reciprocate on a lightreceiving element, showing the deflected beams of light along with thetrajectory thereof.

[0039] In FIG. 2, reference symbol 101 denotes the light receivingelement and reference symbols 102 and 103 denote the deflected beams oflight, while reference symbol 104 denotes the trajectory of thedeflected beams of light.

[0040] The deflected beam of light 102 moves along the trajectory 104 indirection A. The other deflected beam of light 103 moves along thetrajectory 104 in direction B. Both the deflected beam of light 102 andthe deflected beam of light 103 pass by the light receiving element 101.

[0041] The light receiving element 101 is arranged at a predeterminedposition where it can detect (receive) both the deflected beam of lightmoving forward and the deflected beam of light moving backward. Providedthat the deflected beam of light 102 moving forward and the deflectedbeam of light 103 moving backward passing by the receiving element 101take respective positions that are different from each other at a givenclock time for each, the distance between the positions of the deflectedbeams of light 102, 103 is the distance of displacement.

[0042] With this embodiment, at least either the deflection means or thelight source is controlled by a control means, which will be describedin greater detail hereinafter, in such a way that the displacement isfound within an appropriate range of displacement (predetermineddistance).

[0043] The embodiment will be described in greater detail below.

[0044] Firstly, the positional displacement between the position of thebeam of light moving forward and that of the beam of light movingbackward that are deflected by a deflection means will be discussed.

[0045]FIG. 3 is a schematic cross sectional view of the first embodimentof optical deflector taken along a plane containing beams of lightdeflected by the deflection means.

[0046] In FIG. 3, there are shown a light source 201, a deflection means202, a beam of light 203 emitted from a light source, beams of light204, 205 that are deflected by the deflection means 202 with the largestangle of deflection, the central axis 206 of optical deflection of thedeflection means 202 and the scanning trajectory 207 on plane P that isseparated from the deflection means 202 by distance L (planeperpendicular to the central axis 206 of optical deflection).

[0047] The beam of light 203 emitted from the light source 201 is madeto strike the deflection means 202. A light source that is adapted tomodulation such as semiconductor laser is used for the light source 201.

[0048] The deflection means 202 is provided with a reflection plane sothat it deflects a beam of light within the largest angle of deflectionas indicated by the beams of light 204, 205 as it is driven to move. Thelargest angle of deflection is denoted by θ.

[0049] In the following description, it is assumed that the reflectedbeam of light is found on the central axis 206 of optical deflectionwhen the deflection means 202 is not driven to move.

[0050] The deflection means 202 is driven to rotate around the rotaryaxis and reciprocate. A periodical drive waveform is applied to thedeflection means.

[0051]FIGS. 4A through 4C are graphs illustrating exemplary drivewaveforms that can be applied to the deflection means 202 of the firstembodiment. The horizontal axis represents the duration of applicationand the vertical axis represents the amplitude of the applied signal.FIG. 4A shows a triangular waveform and FIG. 4B shows a saw-edgedwaveform, while FIG. 4C shows a sinusoidal waveform. The deflectionmeans 202 changes the angle of deflection corresponding to the waveformof the applied signal.

[0052] As the deflection means 202 is driven to deflect a beam of lightby applying a periodical signal as shown in any of FIGS. 4A through 4C,the scanning position on the plane P that is separated from thedeflection means 202 by distance L (the movement of a deflected beam oflight on a plane is referred to as scanning hereinafter) reciprocates onthe scanning trajectory 207. The scanning position h (the distance fromthe central axis 206 of optical deflection on the plane P that isseparated from the deflection means 202 by distance L) can be expressedby the equation below;

h=L×tan(θ(t))  (1),

[0053] where θ(t) represents the angle of deflection by which the beamof light is deflected from the central axis 206 of optical deflection ata given clock time.

[0054] It may be assumed here that the light source 201 is operated formodulation and the position of the forwardly moving deflected beam oflight 102 and that of the backwardly moving deflected beam of light 103at a given clock time for each are not displaced from each other.

[0055] However, the scanning characteristic of the deflection means 202can change because of the components change due to temperature changesand the drive means delays. Therefore, if the same waveform is appliedto the deflection means at the same timing and the light source 201 isoperated for modulation at the same clock time, the timing of deflection(scanning) of the forwardly moving and backwardly moving beams of lightmay be displaced and hence the position of the forwardly movingdeflected beam of light 102 and that of the backwardly moving deflectedbeam of light 103 at the given clock time for each may be displaced fromeach other on the light receiving element 101.

[0056] With this embodiment, it is hence possible to sense a change inthe displacement of the scanning positions of the forwardly moving andbackwardly moving beams of light due to changes of various factorsrelating to the embodiment including environmental factors (changes inthe scanning timings) by detecting a relative displacement between theposition of the forwardly moving deflected beam of light and that of thebackwardly moving deflected beam of light at a given clock time foreach.

[0057] Note that, while the positions of the deflected beams of light102, 103 change when the largest angle of deflection θ changes due tovarious environmental factors, no relative positional displacement ofthe two deflected beams of light occurs so long as the scanning timingdoes not change. Therefore, it is possible to detect the relativedisplacement of the scanning positions of the forwardly moving andbackwardly moving deflected beams of light without being influenced bythe change in the largest angle of deflection θ.

[0058] Now, the method of detecting the relative displacement of thescanning positions of the forwardly moving and backwardly movingdeflected beams of light by means of a light receiving element will bedescribed below.

[0059] First, a technique of generating modulated spots to be used fordetecting the position of a deflected beam of light by means of a lightreceiving element will be discussed.

[0060] The light receiving element 101 is arranged on the scanningtrajectory 207 located on the plane P that is separated from thedeflection means 202 by distance L. Any position may be selected for thelight receiving element 101 so long as it is found on the scanningtrajectory 207. For the purpose of simplicity of description, assumehere that the light receiving element 101 is arranged substantially atthe center of the scanning area.

[0061] In order to detect the relative displacement of the scanningpositions of the forwardly moving and backwardly moving deflected beamsof light at a given clock time for each, a technique of deflecting amodulated beam of light that is obtained by turning on and off the lightsource to form regions where luminance of light (quantity of light)shows distribution (to be referred to as modulated spot hereinafter) bymeans of the light receiving element 101 and gauging the positional gapbetween the modulated spots will be employed. More specifically, asshown in FIG. 2, the light source is turned on and off once at a givenclock time within a period of time when a beam of light is caused toforwardly scan on the light receiving element 101 and also once within aperiod of time when a beam of light is caused to backwardly scan on thelight receiving element 101 to produce spots (high luminance spotsgenerated by a scanning beam of light) 102, 103 respectively on theforward moving path and the backward moving path.

[0062] As a result, it is possible to detect the positions of thedeflected beams of light at a given clock time for each by observing thedistribution of the total quantity of the electric charge induced bylight on the light receiving element 101.

[0063] Thus, the relative positional displacement of the modulated spotscan be gauged by means of the light receiving element 101 that isadapted to output a signal that allows the gap between the spots 102,103 to be gauged on the light receiving element 101.

[0064] Second, the light receiving element 101 that is adapted to outputa signal that allows the gap to be gauged on the element itself will bediscussed.

[0065] The light receiving element 101 of this embodiment is required todetect the position of each modulated and deflected beam of light aspositional information and also the gap separating the positions of twodeflected beams of light.

[0066] A line sensor (image sensor) comprising a plurality of lightreceiving regions 105 can be used for the light receiving element 101 ofthis embodiment. Such an arrangement requires that each light receivingregion comprises a light receiving element section that operates asphotoelectric transducer, an accumulating section for accumulating theelectric charges that are obtained as a result of photoelectricconversion and a transfer section for transferring the accumulatedelectric charge.

[0067] Then, the position of a deflected beam of light can be accuratelyidentified because each of the plurality of light receiving regions candetect the quantity of the deflected beam of light.

[0068] Then, it is not necessary to transfer the accumulated electriccharge at a high rate corresponding to the scanning speed. Rather, theelectric charge can be transferred at a lower rate after the formationof modulated spots on the forward and backward moving paths on the lightreceiving element 101. Therefore, the relative positional displacementof the modulated spots can advantageously be detected if the scanningspeed v is raised on the light receiving element 101 (e.g., if theperiod of application of a waveform is reduced).

[0069] When such a light receiving element is used, the distribution ofquantity of light in each of the modulated spots on the light receivingelement 101 is accumulated in the form of electric charge and output aspositional information on each of the plurality of light receivingregions 105. Thus, this embodiment is free from the problem of a reducedaccuracy of detection that arises to the method of directly detectingthe scanning (deflection) timing because of changes in the delay of thedetection circuit. Hence, this embodiment can highly accurately detectthe relative positional displacement of the modulated spots.

[0070] This embodiment employs a method of detecting the forwardlymoving modulated and deflected beam of light and the backwardly movingmodulated and deflected beam of light by means of a light receivingelement and gauging the change in the timing of forward scanning andbackward scanning as positional information by referring to therespective scanning positions at a given clock time for each. Therefore,there does not arise any problem of lowered detection accuracy that canbe caused by a delay in the detection circuit of the prior art and henceit is possible to highly accurately detect any change in the scanningcondition.

[0071] Next, the method of controlling the detected relative positionaldisplacement between of the forwardly moving and backwardly movingdeflected beams of light so as to keep it to a constant value will bediscussed.

[0072]FIG. 5 is a schematic block diagram of this embodiment of opticaldeflector, illustrating the control means thereof.

[0073] In FIG. 5, there are shown a deflected beam of light 208, amodulation signal generation means 301 for the light source 201, amodulation signal 305 for the light source 201, a detection signal 306from the light receiving element, a signal conversion means 302, ascanning position displacement signal 307, a control signal generationmeans 303, a control signal 308 for the deflection means 202, a drivemeans 304 for the deflection means 202, a drive signal 309 for thedeflection means 202 and a modulation control signal 310 for the lightsource 201.

[0074] The light source 201 is driven to turn on and off repeatedly (formodulation) by a modulation signal 305 from the modulation signalgeneration means 301 that operates to turn on and off the light source201 with a predetermined period. The modulated beam of light 203 isdeflected by the deflection means 202 and detected by the lightreceiving element 101. The modulation signal 305 is generated so as todetect modulated spots 102, 103 respectively in the forward scanningdirection and in the backward scanning direction. The information on themodulated spots in the forward and backward scanning directions detectedby the light receiving element 101 is transmitted to the signalconversion means 302 as output signal 306.

[0075] The signal conversion means 302 computationally determines therelative positional displacement between the scanning positions of theforwardly moving modulated beam of light and the backwardly movingmodulated beam of light on the basis of the detection signal 306 fromthe light receiving element and outputs a scanning position displacementsignal 307 that represents the relative displacement of the scanningpositions of the modulated and deflected beams of light.

[0076] The control signal generation means 303 modifies either thecontrol signal 308 for the deflection means 202 or the modulationcontrol signal 310 for the light source 201 in such a way that therelative positional displacement of the scanning positions of themodulated and deflected beams of light becomes equal to a predeterminedvalue (e.g., 0) on the basis of the scanning position displacementsignal 307.

[0077] The control signal 308 of the deflection means 202 is adapted tochange the rate at which the movable plate swings in order to change thetiming of deflection of the mirror (movable plate) that belongs to thedeflection means 202.

[0078] The drive means 304 selects a timing or period for the drivesignal according to the control signal 308 and applies the drive signal309 to the deflection means 202, or the deflector.

[0079] The modulation control signal 310 for the light source 201 isadapted to adjust the timing or period of the modulation signal 305 sothat the modulation of light may be in harmony with the deflectiontiming of the mirror (movable plate) that belongs to the deflectionmeans.

[0080] It may be so arranged to change either the control signal 308 forthe deflection means 202 or the modulation control signal 310 for thelight source 201 or both of them.

[0081] Thus, this embodiment of optical deflector can control therelative positional displacement between the forwardly scanning beam oflight and the backwardly scanning beam of light so as to keep it aconstant level by detecting the modulated and deflected beams of lightby means of a light receiving element.

[0082] Thus, by using the above described detection/control method,there is provided a method of driving an optical deflector having adeflection means for deflecting a modulated beam of light from a lightsource, the method comprising:

[0083] a gauging step of gauging the distance between the position of adeflected beam of light moving on a light receiving element in adirection and the position of another deflected beam of light moving inthe opposite direction; and

[0084] a control step of controlling either the light source or thedeflection means so as to make the distance take a predetermined valueby a control means.

[0085] This embodiment is not limited to the above describedconfiguration and may be modified in various different ways as will bedescribed below.

[0086] For instance, it may be so arranged as to transfer theaccumulated electric charge after generating a forwardly movingmodulated spot on the light receiving element 101, generate a backwardlymoving modulated spot after the transfer and subsequently resume thetransfer of electric charge. With this arrangement, the scanningoperation of the deflection means needs to be stabilized within a shortperiod of time (which is at least twice as long as the time required fortransferring the electric charge). Then, the forwardly moving modulatedspot and the backwardly moving modulated spot can be detected separatelyand hence it is possible to use a simplified algorithm to detect thepositional displacement.

[0087] While zero is used above as exemplary value for defining therelative positional displacement between the forwardly moving modulatedspot and the backwardly moving modulated spot for this embodiment,another appropriately selected value may alternatively be used. Then,the forwardly moving modulated spot and the backwardly moving modulatedspot are separated from each other so that they may be detected withease.

[0088] While the light receiving element 101 is arranged on the scanningtrajectory in the above-described embodiment, it may alternatively bearranged at some other position and a mirror may be additionallyprovided so as to reflect the deflected beams of light moving on thescanning trajectory. Then, the light receiving element 101 detects thedeflected beams of light that have been reflected by the mirror. Theprovision of a mirror alleviates the positional restrictions imposed onthe light receiving element 101 so that an optical deflector of thisembodiment can be downsized.

[0089] While only a single light source is used in this embodiment, thepresent invention is applicable to an optical deflector having aplurality of light sources. Only one of the plurality of light sourcesmay be used for the purpose of the invention.

[0090] Any light source that is adapted to modulate the beam of lightemitted from it can be used for this embodiment. Examples of such lightsource include semiconductor lasers, LEDs, solid lasers and gas lasershaving a modulation means such as AOM.

[0091] While this embodiment is described above in terms ofone-dimensional optical scanning where a forwardly moving deflected beamof light and the backwardly moving deflected beam of light pass along asame trajectory, the present invention is also applicable to so-calledtwo-dimensional optical scanning where the backwardly moving deflectedbeam of light proceeds along a trajectory that is perpendicularlyseparate from the trajectory along which the forwardly moving deflectedbeam of light proceeds.

[0092] This embodiment is described above in terms of deflected beams oflight adapted to one-dimensional scanning. However, this embodiment canbe used for an exposure device adapted to emit light onto thecylindrical photosensitive body of an electrophotography type imageforming apparatus so as to produce an electrostatic latent image bytwo-dimensionally scanning the surface of the photosensitive body bymeans of deflected beams of light when the deflected beams of light aremade to scan the revolving cylindrical photosensitive body along thelongitudinal direction thereof.

[0093] This embodiment can also be used for a projection type imagedisplay apparatus such as a projector when the deflected beams of lightare made to scan two-dimensionally.

[0094] In such an image forming or image display apparatus, the light isturned on and off corresponding to the pixels of the image beingproduced. The size of each pixel is not particularly limited. In otherwords, it is defined according to the image to be produced by theapparatus. In each pixel, not only the actual scanning spot diameter butalso its shape change as a function of the scanning distance because thescanning spot moves in one direction while the light source is on,though the change depends on the scanning speed. If a light source whosequantity of light differs between the center of light emitting point andthe peripheral area (e.g., to show a Gaussian distribution) is used, thepractical pixel size may be regarded to be equal to that of the regionwhere a large quantity of light is found (e.g., a half of the largestquantity of light or 1/e²) is found regardless if the pixel moves in onedirection while the light source is on. In the case of a projector, forexample, that produces an image that human eyes can directly watch, thesize of the pixel that moves while the light source is on may be definedas such, taking the human vision into consideration.

[0095] With this embodiment of optical deflector, the relativepositional displacement of the forwardly moving modulated pattern andthe backwardly moving modulated pattern can be eliminated on theprojection surface (that is scanned by beams of light) without beinginfluenced by the change in the scanning timing of the deflection means202 when the relative positional displacement of the forwardly movingdeflected beam of light and the backwardly moving deflected beam oflight that are moving on the light receiving element is so controlled asto show a constant value.

[0096] Thus, when this embodiment is used for an exposure device adaptedto emit light onto the photosensitive body of an electrophotography typeimage forming apparatus or a display apparatus of the above describedtype that is adapted to display an image on a two-dimensional displayscreen, it can display a desired image by using both a forwardly movingscanning beam of light and a backwardly moving scanning beam of light toimprove the exposure rate or the display speed, whichever appropriate.

[0097] (Second Embodiment)

[0098] This embodiment differs from the first embodiment in terms of themethod for identifying the position (central position) of each of themodulated and deflected beams of light (modulated spots) on the lightreceiving element 101 comprising a plurality of light receiving regions105. Otherwise, this embodiment is identical with the first embodiment.

[0099]FIGS. 6A, 6B and 6C are schematic illustrations of the secondembodiment.

[0100]FIG. 6A shows the positional relationship of the position on thelight receiving element 101 where a modulated spot is formed and theplurality of light receiving regions 105. Note that the scanningdirection is horizontal on FIG. 6A. For the purpose of simplicity,assume here that there is a single modulated spot on the light receivingelement 101. For the purpose of the invention, the process that isdescribed below may be repeated for the number of times that is equal tothe number of spots that is involved in the optical deflector.

[0101]FIG. 6B is a graph illustrating the distribution of quantity oflight irradiated onto the light receiving element 101 when thepositional relationship of FIG. 6A is applicable. In FIG. 6B, thehorizontal axis represents the position on the light receiving elementand the vertical axis represents the quantity of light. The modulatedspot shows a distribution pattern that is symmetrical relative toposition C where the quantity of light is largest (which may be aGaussian distribution pattern in which the center is light and theperipheral area is dark).

[0102]FIG. 6C shows the signals that are detected in the plurality oflight receiving regions 105 when the positional relationship of FIG. 6Ais applicable. In FIG. 6C, the horizontal axis represents the positionof each of the plurality of light receiving regions that corresponds toa selected position on the light receiving element and the vertical axisrepresents the magnitude of the detected signal. In the followingdescription, the width of each of the plurality of light receivingregions that runs in the scanning direction is assumed to be w and thequantity of light detected on the n-th light receiving region from theleft end in FIG. 6B is assumed to be Pn.

[0103] Two position identifying methods will be described below byreferring to FIGS. 6A through 6C.

[0104] With the first position identifying method, a positionalcoordinate system is defined on the basis of the light receiving regionthat obtained the largest quantity of light in the plurality of lightreceiving regions. For example, in the case of FIG. 6C, the 5th lightreceiving region takes the role of the basis of a positional coordinatesystem and the center of the modulated spot is detected so as to belocated at the position of [(5−0.5)×w].

[0105] The detection accuracy is determined by the relationship betweenthe scanning velocity v on the light receiving element 101 and the widthw of each of the plurality of light receiving regions 105 that runs inthe scanning direction. If the scanning velocity v is constant, thedetection accuracy rises as the width w of each of the plurality oflight receiving regions that runs in the scanning direction is reduced.

[0106] With the first position identifying method, a simple process canbe used to identify a particular position on the light receiving elementso that both the time necessary for the process and the load borne bythe components responsible for the process can be reduced. Additionally,the detection accuracy can be improved by reducing the width w of eachof the plurality of light receiving regions 105 that runs in thescanning direction.

[0107] With the second position identifying method, a position on thelight receiving element is identified on the basis of the distributionof detection signals obtained on the plurality of light receivingregions. The distribution of quantity of light of the modulated spotdetected on the light receiving element can be regarded to besymmetrical relative to position C where the quantity of light islargest. Thus, if the position C where the quantity of light is largestdoes not agree with the center of one of the plurality of lightreceiving regions 105 or the boundary of one of the light receivingregions 105, the detection signals from the light receiving regions 105are arranged asymmetrically (see the detection signals of FIG. 6C as anexample). Using this phenomenon, the positional coordinate of each ofthe light receiving regions [(n−1)×w] is multiplied by the quantity ofreceived light Pn and the products of the multiplications are added toobtain the total sum [Σ(n−1)×w×Pn]. Then, the total sum is divided bythe total quantity of received light [ΣPn] in the light receiving regionwhere the spot exists ([Σ(n−1)×w×Pn]/ΣPn) to identify the positionalcoordinate. Thus, it is now possible to detect the center of the spotwith a level of error smaller than the width w of each of the pluralityof light receiving regions. With this method, it is preferable that thedeflected beam of light extends on more than one plurality of lightreceiving regions 105 from the viewpoint of identifying the position ofthe deflected beam of light. This method can be used when if theposition C where the quantity of light is largest does not agree withthe center of one of the plurality of light receiving regions 105 or theboundary of one of the light receiving regions 105.

[0108] With the above described second method, it is possible to providea resolution that is smaller than the width w of each of a plurality oflight receiving regions 105 so that the position of the modulated spotcan be identified highly accurately if a light receiving element 101 inwhich the width w of each of a plurality of light receiving regions 105is relatively large (smaller than the width of the modulated spot to bedetected) is used.

[0109] (Third Embodiment)

[0110] This embodiment differs from the first and second embodiments inthat the light receiving element 101 has a plurality of light receivingregions 105 that are two-dimensionally arranged. Otherwise, thisembodiment is identical with the first and second embodiments.

[0111]FIG. 7 is a schematic illustration of a plurality of lightreceiving elements that the light receiving element of the thirdembodiment includes.

[0112] As shown in FIG. 7, the light receiving element 101 has aplurality of light receiving regions 105 that are arrangedtwo-dimensionally. In other words, the light receiving regions 105 havea square profile and are arranged in rows and columns.

[0113] When a plurality of light receiving regions 105 are arrangedtwo-dimensionally as in the case of this embodiment, the distribution ofquantity of light of a modulated spot can be detected two-dimensionallyso that the profile of the modulated spot can be grasped accurately anda large quantity of data can be used for selecting either of theposition identifying methods described above in terms of the secondembodiment.

[0114] Thus, the position of the modulated spot can be identified highlyaccurately by using this embodiment.

[0115] The light receiving element 101 of this embodiment can berealized by using a general purpose CCD area sensor or CMOS area sensorthat finds applications in imaging operations so that it is notnecessary to design a particular sensor for the light receiving element101. Thus, this embodiment can be realized at low cost.

[0116] While the light receiving element of this embodiment has aplurality of light receiving regions having a square profile andarranged in rows and columns in the above description, it mayalternatively have a configuration where a plurality of light receivingregions are arranged to show a honeycomb, a configuration where the rowsor the columns of light receiving regions are alternately indented andoffset in the direction of the trajectory or in a directionperpendicular to the trajectory, whichever appropriate, or aconfiguration where the light receiving regions have a circular,parallelogramic, triangular, rhombic, trapezoidal or polygonal profile.

[0117] (Fourth Embodiment)

[0118] In this embodiment, deflected beams of light entering the lightreceiving element is converged by a lens. Otherwise, this embodiment isidentical with the preceding first through third embodiments.

[0119]FIG. 8 is a schematic cross sectional view of the fourthembodiment of optical deflector taken along a plane containing a beam oflight deflected by the deflection means. In FIG. 8, reference symbol 209denotes a lens.

[0120] The lens 209 is arranged at a position separated from thedeflection means by distance L. The light receiving element 101 isarranged at a position separated from the lens 209 by a distance equalto the depth of focus f of the lens 209. The beam of light deflected bythe deflection means 202 shows a reduced width (diameter) when it isconverged by the lens to form a modulated spot. Additionally, the beamof light is further deflected by the lens so that the center of themodulated spot is shifted from that of the modulated spot of the firstembodiment.

[0121] The relationship between the scanning position h and the angle ofdeflection θt at time t is expressed by the formula below.

h=f×tan(θt)  (2)

[0122] From the equation (2), it will be seen that the scanning velocityv on the light receiving element 101 can be raised by selecting a largedepth of focus f for the lens 209 regardless of the distance L. Then,the rate of change of the relative positional displacement of theforwardly moving modulated spot and the backwardly moving modulated spotis raised to improve the detection accuracy.

[0123] Additionally, the positions of the modulated spots do not dependon the distance L. Therefore, the arrangement of the related componentsis facilitated and the embodiment can be downsized when a small value isselected for the distance L and a lens 209 having a large depth of focusf is used.

[0124] The lens may be used as a component of this embodiment of opticaldeflector. Alternatively, the lens may be integrally arranged with thelight receiving element.

[0125] The light receiving element 101 can accurately identify thepositions of the deflected beams of light when the deflected beams oflight are converged onto the light receiving element 101 by means of alens 209 as in this embodiment. Additionally, the entire opticaldeflector can be downsized.

[0126] (Fifth Embodiment)

[0127] This embodiment differs from the first embodiment in that itcomprises a deflection means 202 that utilizes a resonance phenomenon.Otherwise, this embodiment is identical with the first embodiment.

[0128] When a resonance type deflector is used for the deflection means202, it is possible to provide a wide angle of deflection by making themechanical resonance frequency fc of the resonance type deflector andthe drive frequency fd agree with each other if the drive energy is notraised. However, the mechanical resonance frequency fc of the deflectorcan change remarkably as a function of the change in the environmentalfactors of the deflector including ambient temperature and therefore thescanning (deflecting) timing of the deflector 202 changes.

[0129] Therefore, it is necessary to control the resonance frequency fcof the resonance type optical deflector and the drive frequency fd so asto make them agree with each other.

[0130]FIGS. 9A and 9B are graphs illustrating the frequencycharacteristics of the resonance type deflector of the fifth embodimentof optical deflector.

[0131] In FIG. 9A, the horizontal axis represents the frequency fd ofthe drive signal for swinging the resonance type deflector and thevertical axis represents the amplitude of the change in the angle ofdeflection (swinging angle) (largest angle of deflection θ) of theresonance type deflector. In FIG. 9A, the frequency that provides thelargest value for the largest angle of deflection θ is the resonancefrequency fc (in an ideal case where any delay in the drive circuitand/or some other circuits does not need to be taken intoconsideration).

[0132] In FIG. 9B, the horizontal axis represents the frequency fd ofthe drive signal for swinging the resonance type deflector (just likethe horizontal axis of FIG. 9A) and the vertical axis represents thephase delay from the synchronizing signal of the drive frequency fd.Note that the origin (0 deg) on the horizontal axis of the phase delaycan change depending on how the synchronizing signal of the drivefrequency fd is generated.

[0133] As seen from FIGS. 9A and 9B, the phase changes as the drivefrequency fd and the resonance frequency fc vary and hence the timing ofscanning changes in a resonance type optical deflector.

[0134] The relationship between the two graphs is maintained when theresonance frequency fc of the deflector is constant. The relationshipbetween the graphs of FIGS. 9A and 9B is maintained if the resonancefrequency fc changes (the profiles of the curves including the slopesand the widths of the curves are held similar) and only the parameter ofthe drive frequency fd represented by the horizontal axes of FIGS. 9Aand 9B changes.

[0135] Thus, it is possible to make the drive frequency fd and theresonance frequency fc agree with each other by driving the resonancetype deflector with a drive frequency fd that constantly maintains (bychanging the drive frequency to maintain) a same scanning timing(phase).

[0136] The influence on the phase (scanning timing) increases as thevalue of the drive efficiency (Q value of resonance) rises, if thefrequency difference remains the same. Therefore, it is necessary tochange the frequency at a smaller pitch.

[0137] Thus, in this embodiment, it is possible to constantly maintainthe same scanning timing by detecting the relative positionaldisplacement of the forwardly moving modulated beam of light and thebackwardly moving modulated beam of light and controlling the drivefrequency fd so as to make it follow the resonance frequency fc of theresonance type optical deflector.

[0138]FIGS. 10A and 10B are graphs illustrating drive signal 309 of thedeflection means 202 of the resonance type optical deflector of thefifth embodiment and the change with time of the angle of deflection atthe time when the drive signal 309 is applied.

[0139]FIG. 10A shows the temporal change of the amplitude (e.g., of thevoltage) of the drive signal 309 of the deflection means 202. In FIG.10A, the horizontal axis represents time and the vertical axisrepresents the amplitude of the drive signal 309.

[0140] In FIG. 10B, the horizontal axis represents time (just like thehorizontal axis of FIG. 10A) and the vertical axis represents the angleof deflection θt of the deflection means 202 at time t.

[0141] If the drive signal shows a sinusoidal waveform and the drivefrequency fd and the resonance frequency fc agree with each other asshown in FIG. 10A, a phase delay of 90 degrees occurs to the change ofthe angle of deflection θt as shown in FIG. 10B. A phase offset of 180degrees can occur depending on the selection of the sense of deflectionof the deflection means 202 (the definition of the positive side and thenegative side for a slope inclined in a direction relative to the rotaryaxis).

[0142] When a resonance type optical deflector is used, the angle ofdeflection θt changes with time to show a sinusoidal waveform even ifthe waveform of the drive signal is not sinusoidal but triangular,rectangular or saw-edged.

[0143] Therefore, since the scanning (deflection) time of the forwardlymoving beam of light and that of the backwardly moving beam of light areequal to each other, the time that can be used for the modulatingoperation of the light source 201 is shortened (to less than a half ofthe available time) and the efficiency of the use of light is reduced ifonly one of the scanning beams of light is utilized. The modulatingoperation of the resonance type optical deflector needs to be conductedby using both the forwardly moving beam of light and the backwardlymoving beam of light in order to avoid the above identified problem.

[0144] With this embodiment, it is possible to maintain the relativedisplacement of the scanning position of the forwardly moving scanningbeam of light and that of the backwardly moving scanning beam of lightby using a resonance type deflector for the deflection means 202. Thus,a resonance type deflector that can produce a large angle of deflectionat a low power consumption rate can be used for an application whereboth a forwardly moving scanning beam of light and a backwardly movingscanning beam of light are utilized. As a result, it is possible toprovide an optical deflector whose power consumption rate is low andefficiency of utilization of light is high.

[0145] While the drive frequency fd of this embodiment is made to agreewith the resonance frequency fc in the above description, it is alsopossible to make the drive frequency fd and the resonance frequency fcto always show a constant difference and hence a certain constantrelationship. If such is the case, the ratio of the change in thescanning timing relative to the frequency displacement becomes small tofacilitate the operation of controlling the drive frequency fd so as tomake it follow the resonance frequency fc.

[0146] (Sixth Embodiment)

[0147] This embodiment differs from the first through fifth embodimentsin terms of the method for generating modulated spots and detectingtheir relative positional displacement and the method for detecting therelative positional displacement. Otherwise, it is identical with thepreceding first through fifth embodiments.

[0148]FIGS. 11A and 11B are schematic illustrations of a method ofgenerating modulated spots by the sixth embodiment of optical deflector.

[0149]FIG. 11A shows a waveform that the drive signal 309 to be appliedto the deflection means 202 can take. In FIG. 11A, the horizontal axisrepresents time and the vertical axis represents the amplitude of theapplied signal. While the waveform of the drive signal is triangular, itis illustrated only as example.

[0150]FIG. 11B shows the modulation signal 305 to be used for modulating(turning ON and OFF) the light source 201. In FIG. 11B, the horizontalaxis represents time (just like the horizontal axis of FIG. 11A) and thevertical axis represents the pattern of the modulation signal 305. Notethat the modulation signal 305 is usually an OFF signal but it turns tobe an ON signal when the light source 201 is modulated to generate amodulated spot on the light receiving element 101.

[0151] The modulation signal 305 produces an ON signal once for eachtime period, during which the deflection means 202 is driven to makeboth a forwardly moving beam of light and a backwardly moving beam oflight scan on a plane containing the light receiving element 101, andgenerates modulated spots 102, 103, which are separated from each other,on the light receiving element 101.

[0152] While the drive signal has a triangular waveform in the abovedescription, some other waveform may alternatively be used. FIGS. 12Aand 12B schematically illustrate a drive signal having a sinusoidalwaveform that can be used for generating modulated spots (both thevertical axes and the horizontal axes of FIGS. 12A and 12B are identicalwith their counterparts of FIGS. 11A and 11B).

[0153] While only a single forwardly or backwardly moving modulated spotis generated in the above description, a plurality of forwardly orbackwardly moving modulated spots may be alternatively be generated.When a plurality of modulated spots are detected by the light receivingelement 101, the relative positional displacements are gauged and theaverage of the positional displacements is determined and used toimprove the accuracy of detection.

[0154] When this embodiment of optical deflector is used for theexposure device of an image forming apparatus or an display device andthe relative positional displacement of the forwardly moving modulatedspot and the backwardly moving modulated spot is made to show apredetermined value (which may be equal to 0), they should be controlledto operate for exposure or display properly. Then, a high quality imageis formed by using both forward scanning and backward scanning.

[0155] (Seventh Embodiment)

[0156] This embodiment differs from the first through sixth embodimentsin that the optical deflector projects deflected beams of lighttwo-dimensionally on the light receiving surface. Otherwise, it isidentical with the preceding first through sixth embodiments.

[0157]FIG. 13 is a schematic illustration of the seventh embodiment ofoptical deflector.

[0158] In FIG. 13, there are shown a second deflection means 211, ascanning trajectory 210 on the reflection plane of the second deflectionmeans 211 formed by the deflection means 202, beams of light 212deflected by the second deflection means 211, the scanning area 214 on aplane 213 in which deflected beams of light scan, and the trajectory 215of scanning beams of light on the plane 213.

[0159] Note that the arrangement for controlling the operation of theoptical deflector as shown in FIG. 5 is not illustrated in FIG. 13.

[0160] Each of the deflection means 202 and the second deflection means211 is adapted to deflect each beam of light both horizontally andvertically. Therefore, the deflected beams of light produced by thedeflection means cover a two-dimensional region.

[0161] The deflection means 202 and the second deflection means 211 haverespective deflection velocities that are different from each other.More specifically, when the two deflection means are compared with eachother in FIG. 13, the deflection means 202 deflects beams of light atrelatively high speed (high frequency), whereas the second deflectionmeans 211 deflects beams of light at relatively low speed (lowfrequency). The speed relationship may be inverted.

[0162] The deflection means that deflects beams of light at relativelyhigh speed can display highly fine images when a resonance typedeflector is used. This is because a resonance type deflector is adaptedto high speed deflecting operations.

[0163] The beam of light 203 modulated by and emitted from the lightsource 201 is deflected by the deflection means 202, the largest angleof deflection being defined by beam of light 204 and beam of light 205(the largest angle of deflection θ). The second deflection means 211deflects the beams of light that scan the reflection plane along thetrajectory 210 to produce beams of light 212 that scan the plane 213arranged at a selected position so as to define a scanning area asindicated by 214. Note that reference symbol 215 denotes theschematically illustrated trajectory of scanning beams of light withinthe scanning area 214 on the plane 213.

[0164] The light receiving element 101 is placed at an appropriateposition in the scanning area 214. More specifically, it is placed on ahorizontal part of the scanning trajectory.

[0165]FIG. 14 is a schematic illustration of the light receiving element101 arranged within the scanning area 214 and the display region of thelight receiving element 101 of this embodiment.

[0166] In FIG. 14, reference symbol 220 denotes the display region to beused for forming images.

[0167] The scanning area 214 includes a display region 220 and a regionwhere the light receiving element 101 is arranged. As deflected beam oflight 212 starts scanning from scanning point S1, it moves back andforth in the horizontal scanning direction X to gradually scan fromupper part of the scanning area to lower part of the area along thevertical scanning direction Y. When the deflected beam of light 212 getsto scanning point S2, it is returned to scanning point S1 and repeatsthe same scanning cycle.

[0168] It is so arranged that the deflected beam of light 212 moves onthe light receiving element 101 that is placed on the scanning line 215.

[0169] With this embodiment of optical deflector that is applied to atwo-dimensional image forming apparatus comprising a resonance typedeflector, the relative positional displacement of the modulated beamsof light is detected by the light receiving element 101 and controlledto show a predetermined desired value while the modulated beams of lightare forming an image. Thus, it is possible to display a high qualityimage by means of a resonance type deflector that produces a forwardlymoving beam of light and a backwardly moving beam of light.

[0170] While the light receiving element 101 is arranged within thescanning area 214 in the above description of the embodiment, scanningbeams of light, or deflected beams of light between the deflection means202 and the second deflection means 211, may be taken out from thescanning area 214 for detection by means of a reflector mirror.

[0171] While the region where the light receiving element 101 isarranged and the display region 220 are separated from each other in theabove description of the embodiment, a region for arranging the lightreceiving element 101 or a reflector mirror to be used by the lightreceiving element 101 to detect modulated spots may be provided withinthe display region 220 so long as the light receiving element 101 or thereflector mirror does not visually adversely affect the displayed image.

[0172] The deflected beam of light 212 may be so adapted that it becomesbright only when it moves on the light receiving element 101. In otherwords, the deflected beam of light 212 is required to be bright at leaston the part of the trajectory located on the light receiving element101.

[0173] (Eighth Embodiment)

[0174] This embodiment of optical deflector is characterized in that itcomprises a light receiving element 101. Otherwise, it is identical withthe first through seventh embodiments.

[0175] Like the seventh embodiment, this embodiment of optical deflectoris applied to an image forming apparatus and also comprises twodeflection means that are driven to produce respective deflected beamsof light for horizontal scanning and vertical scanning to display atwo-dimensional image. Only the arrangement that differentiates thisembodiment from the seventh embodiment will be discussed below.

[0176]FIG. 15 is a schematic illustration of this embodiment of opticaldeflector.

[0177] In FIG. 15, there are shown a frame body 213, a scanning region214 provided on the plane of the frame body, a display region 220 andthe trajectory 215 of scanning beams of light in the display region.

[0178] The light receiving element 101 is arranged in the scanningregion 214 on the frame body. Thus, the plane containing the displayregion can be separated from the plane carrying the light receivingelement.

[0179] Additionally, because the distance L from the deflection means202 to the light receiving element 101 and the distance from the centralaxis 206 of optical deflection to the position of the light receivingelement 101 can be fixed, the timing of generating a modulation patterncan be computed with ease on the basis of the arrangement of the lightreceiving element.

[0180] This embodiment of optical deflector can be applied to an imagedisplay apparatus such as a projector of the front type with which theviewer watches the image formed in the display region 220 from aposition located between the frame body and the image.

[0181] This embodiment allows to freely define the display regionbecause the display region onto which an image is projected can beplaced on any plane. In other words, the projector can be used with anyplane because the plane on which an image is projected for displaying isnot subjected to any restrictions.

[0182] This embodiment of optical deflector can be applied to an imagedisplay apparatus such as a rear projector when it is so arranged thatthe viewer watches the image formed in the display region 220 from theside opposite to the display surface of the display region 220.

[0183] Additionally, this embodiment can also be applied to an imagedisplay apparatus of the type adapted to display an image directly onthe retinas of the viewer or a head-mount display type image formingapparatus.

[0184] While the frame body 213 is not an indispensable component ofthis embodiment of optical deflector, the provision of a frame body 212is preferable because the light receiving element 101 can be alignedwith ease when it is rigidly secured to the frame body 213.

[0185] The use of a frame body 213 is also preferable for defining thedisplay region 220. Therefore, a frame body 213 on which the lightreceiving element 101 is arranged may be defined as an indispensablecomponent of this embodiment.

[0186] While the term “modulation pattern” is used in the abovedescription of the preferred embodiments, it will be paraphrased to“modulation pattern for detection” in the following description in orderto discriminate it from a modulation pattern for drawing an image thatis used for image formation.

[0187] As for the scanning direction of the deflection means, scanningfrom left to right on the light receiving element 101 in FIG. 2 isdefined to be forwardly moving direction.

EXAMPLE 1

[0188] In Example 1, an optical deflector according to the invention isused for an exposure device adapted to emit light onto thephotosensitive body of an electrophotography type image formingapparatus.

[0189]FIG. 16 is a schematic illustration of the configuration of thedevice of Example 1.

[0190] In FIG. 16, there are shown a light receiving element 101, alight source 201, a deflection means 202, a photosensitive drum 220, theaxis 221 of the photosensitive drum that includes a trajectory ofscanning, a beam of light 203 emitted from the light source 201 andbeams of light 204, 205 that define the largest angle of deflection.

[0191] The beam of light 203 modulated by and emitted from the lightsource 201 is deflected by the deflection means 202 to produce deflectedbeams of light that scan forwardly and backwardly on the axis 221respectively so as to form a desired modulation pattern on thephotosensitive body within a scanning period and expose thephotosensitive body to light. The largest angle of deflection and thearrangement of the light receiving element 101 are so selected as tomake it possible to detect modulated spots on the light receivingelement 101 (detect scanning beams of light outside the photosensitivebody).

[0192] The light source 201 is directly modulated by means of aninfrared semiconductor laser (λ=780 nm). The deflection means 202 is agalvano-mirror driven by a 10 kHz triangular wave. A {fraction (1/7)}inch CMOS image sensor (black and white sensor conforming to the CIFSpecifications) is used for the light receiving element 101.

[0193] The modulation pattern is defined such that modulated spots areformed on the light receiving element 101 as shown in FIG. 2. As shownin FIG. 2, the modulated spot that is scanning forwardly is located leftto the modulated spot that is scanning backwardly and it is so arrangedthat the photosensitive body is exposed properly to the forwardlyscanning beam of light and the backwardly scanning beam of light whenthe relative positional displacement of the modulated spots gets topredetermined value Lg±α, where α is the tolerance for the accuracy offorward scanning and backward scanning of the photosensitive body.

[0194] The above operation is controlled by the arrangement illustratedin FIG. 5.

[0195]FIG. 17 is a flow chart of the operation of this example.

[0196] As the control starts, firstly an initial drive frequency and aninitial modulation pattern for detection are defined (S101). Firstly,the deflection means 202 is driven on the basis of the above informationto generate a modulation pattern for detection.

[0197] The modulation pattern for detection is generated on the lightreceiving element 101 in such a way that it is turned on once for aforward scanning period and once for a backward scanning period (S102).After the generation of the modulated spots, the electric chargeaccumulated in the light receiving element by the modulated spots istransferred in such a way that the electric charge in one of theplurality of light receiving regions is transferred at a time (S103).

[0198] Then, the relative positional displacement of the modulated spotsis computed on the basis of the transferred information (S104) by usingthe method described above by referring to the second embodiment for thecomputation.

[0199] If the computed relative positional displacement is found withinthe predetermined range of Lg±α, the steps from S102 are repeated. If,on the other hand, it is not found within the predetermined range, it isdetermined if the relative positional displacement is greater or smallerthan Lg (S106).

[0200] If the relative positional displacement is greater than Lg, it isbecause the modulation timing is too quick and hence the gap separatingthe forwardly scanning modulated spot and the backwardly scanningmodulated spot is made too large. Therefore, the modulation timing ismade slower (S107).

[0201] If, on the other hand, the relative positional displacement issmaller than Lg, it is because the modulation timing is too slow andhence the gap separating the forwardly scanning modulated spot and thebackwardly scanning modulated spot is made too small. Therefore, themodulation timing is made quicker (S108).

[0202] When shifting the modulation timing, the positional relationshipof the modulation pattern for detection and the modulation pattern onthe photosensitive body is maintained so that only the timing is madequicker or slower.

[0203] Thereafter, the processing operation returns to S102 to repeatthe above steps.

[0204] In this way, the relative positional displacement of theforwardly scanning modulated spot and the backwardly scanning modulatedspot can be held within a predetermined range. Thus, there arises nodisplacement between exposure to the forwardly scanning modulated spotand exposure to the backwardly scanning modulated spot on thephotosensitive body. In other words, the photosensitive body is exposedto light correctly and therefore, it is possible to realize anelectrophotography type image forming apparatus that produces fineimages.

EXAMPLE 2

[0205] Example 2 differs from Example 1 in generation of modulationpattern for detection and the method of transferring the accumulatedelectric charge from the light receiving element. Otherwise, thisexample is identical with the preceding example.

[0206] Unlike in Example 1, a modulation pattern for detection is notgenerated for forward scanning and for backward scanning successivelyand the value of Lg is equal to 0 in this example. In other words, ifthe photosensitive body is exposed properly to the forwardly scanningmodulated spot and the backwardly scanning modulated spot (and hencethere is no relative positional displacement thereof), the position ofthe modulation pattern for detection for forward scanning agrees withthat of the modulation pattern for detection for backward scanning.

[0207]FIG. 18 is a flow chart of the operation of Example 2.

[0208] After initialization (S201), a single ON signal is generated asmodulation pattern for detection in a forward scanning period and amodulated spot is generated on the light receiving element 101 (S202).Thus, only a modulated spot is generated at position 102 in FIG. 2. Theelectric charge accumulated by the modulated spot in the forwardscanning period is transferred on a region by region basis (S203). Then,a single ON signal is generated as modulation pattern for detection in abackward scanning period and the accumulated electric charge istransferred (S204 and S205).

[0209] The positional relationship and the relative positiondisplacement of the forwardly scanning modulated spot and the backwardlyscanning modulated spot are computed from the obtained positionalinformation of those spots (S206). If the position displacement is foundwithin the tolerance ±α, the steps from S202 are repeated. If, on theother hand, the positional displacement is not found within thetolerance, the positional of the forwardly scanning modulated spot andthat of the backwardly scanning modulated spot are compared. Note that,the relative positional displacement is defined to be positive when theforwardly scanning modulated spot is found to the left of the backwardlyscanning modulated spot, whereas the relative positional displacement isdefined to be negative when the forwardly scanning modulated spot isfound to the right of the backwardly scanning modulated spot (S208).

[0210] If the positional displacement is positive, it is because themodulation timing is too quick and hence the forwardly scanningmodulated spot is found to the left of the backwardly scanning modulatedspot. Therefore, the modulation timing is made slower (S209).

[0211] If, on the other hand, the relative positional displacement isnegative, it is because the modulation timing is too slow and hence theforwardly scanning modulated spot is found to the right of thebackwardly scanning modulated spot. Therefore, the modulation timing ismade quicker (S210).

[0212] Thereafter, the processing operation returns to S201 to repeatthe above steps.

[0213] In the case of this example, it is possible to separately detectthe forwardly scanning modulated spot and the backwardly scanningmodulated spot on a light receiving element 101 having only a smallregion. Thus, the light receiving element 101 can be downsized to reducethe cost of the entire apparatus.

[0214] A time lag occurs between the generation of the forwardly movingmodulated spot for detection and that of the backwardly moving modulatedspot for detection (time period required for transferring theaccumulated electric charge). Therefore, the arrangement of this examplemay preferably be employed when a resonance type optical deflector whosescanning characteristics would not fluctuate in a short period of timeis used for the deflection means 202.

EXAMPLE 3

[0215] In Example 3, an optical deflector according to the invention isused for a laser scanning projection type display apparatus.

[0216]FIG. 19A is a schematic illustration of the configuration of theapparatus of this example.

[0217] In FIG. 19A, reference symbol 222 denotes a reflector mirror andreference symbol 209 denotes a converging lens. Otherwise, theconfiguration is identical with that of 8th Embodiment.

[0218] The light source 201 is directly modulated by means of a redsemiconductor laser (λ=635 nm). The deflection means 202 drives theresonance type optical deflector having a resonance frequency of 28 kHzby means of a rectangular wave. A CMOS image sensor having 100×100regions (each region is a 10 μm square) is used for the light receivingelement 101. The galvano-mirror of the second deflection means 211 isdriven by means of a saw-edged wave of a frequency of 60 Hz. Themodulated spots on the light receiving element are substantial circleshaving a diameter of about 40 μmø.

[0219] The deflected beam of light reflected by the reflector mirror 222is converged by the lens 209 to form a modulated spot on the lightreceiving element 101.

[0220] Modulation patterns are so defined that modulated spots areformed on the light receiving element 101 in a manner as shown in FIG.19B. Since the apparatus is adapted to two-dimensional scanning, thetrajectory 104 of the forwardly scanning modulated spot (scanning indirection A) and the trajectory 104′ of the backwardly scanningmodulated spot (scanning in direction B) are different from each otheron the light receiving element 101 as shown in FIG. 19B. Therefore, theforwardly scanning modulated spot and the backwardly scanning modulatedspot are separated from each other and hence can be recognized withease. In the instance of FIG. 19B, it is possible to recognize that theupper modulated spot 111 is the forwardly scanning one, whereas thelower modulated spot 111′ is the backwardly scanning one.

[0221] The gap between the two trajectories and their respectiveinclinations change depending on the positional arrangement of the lightreceiving element 101 and the method employed for two-dimensionalscanning (scanning trajectories do not necessarily agree with thehorizontal direction of the displayed image).

[0222] It is so arranged that an image is properly displayed on theprojection surface by forward scanning and backward scanning when thepositional displacement between the forwardly scanning modulated spotand the backwardly scanning modulated spot on the light receivingelement 101 (in terms of horizontal coordinate on the light receivingelement 101) is found within a predetermined range 0±α as shown in FIG.19B, where α is the tolerance of accuracy for forward scanning andbackward scanning for the image being displayed.

[0223] The above operation is controlled by the arrangement illustratedin FIG. 5.

[0224]FIG. 20 is a flow chart of the operation of this example.

[0225] As the control starts, firstly an initial drive frequency and aninitial modulation pattern for detection are defined (S301). Firstly,the deflection means 202 is driven on the basis of the above informationto generate a modulation pattern for detection.

[0226] The modulation pattern for detection is generated in such a waythat it is turned on once for a forward scanning period and once for abackward scanning period (S302). After the generation of the modulatedspots, the electric charge accumulated in the light receiving element istransferred by the modulated spots in such a way that the electriccharge in one of the plurality of light receiving regions is transferredat a time (S303).

[0227] Then, the positional relationship and the relative positionaldisplacement of the modulated spots are computed on the basis of thepositional information on the forwardly scanning modulated spot and thebackwardly scanning modulated spot (S304) by using the method describedabove by referring to the second embodiment for the computation.

[0228] If the computed relative positional displacement is found withinthe predetermined tolerance range of ±α, the steps from S302 arerepeated. If, on the other hand, it is not found within thepredetermined tolerance range, their positions in the horizontaldirection of the light receiving element 101 are compared with eachother. Note that, the relative positional displacement is defined to bepositive when the forwardly scanning modulated spot is found to the leftof the backwardly scanning modulated spot, whereas the relativepositional displacement is defined to be negative when the forwardlyscanning modulated spot is found to the right of the backwardly scanningmodulated spot (S306).

[0229] If the positional displacement is positive, it is because themodulation timing is too quick and hence the forwardly scanningmodulated spot is found to the left of the backwardly scanning modulatedspot. Therefore, the drive frequency is raised so as to delay the phaseof the resonance type optical deflector proceed (S307).

[0230] If, on the other hand, the relative positional displacement isnegative, it is because the modulation timing is too slow and hence theforwardly scanning modulated spot is found to the right of thebackwardly scanning modulated spot. Therefore, the drive frequency israised to make modulation timing quicker (S308)

[0231] Thereafter, the processing operation returns to S302 to repeatthe above steps.

[0232] In this way, the relative positional displacement of theforwardly scanning modulated spot and the backwardly scanning modulatedspot can be held within a predetermined range when a resonance typeoptical deflector is used for the deflection means. Thus, there arisesno displacement between image drawing of the forwardly scanningmodulated spot and that of the backwardly scanning modulated spot onprojection surface so that an image is properly displayed. In otherwords, it is possible to realize a projection type image formingapparatus that produces fine images.

[0233] As two-dimensional scanning beams of light are detected by meansof an area sensor (light receiving element), the modulated spots can beseparated from each other and identified with ease. Additionally, thelight receiving element can be downsized and hence its cost can bereduced.

[0234] The light receiving element 101 is freed from restrictions ofpositional arrangement because of the use of a reflector mirror 222 anda lens 209 so that the entire apparatus can be downsized. Additionally,it is possible to improve the detection accuracy.

[0235] As described above by way of example, the present invention canprovide an optical deflector that does not use a detection means fordetecting the time when a beam of light passes by a predetermined angleof deflection of a deflection means. Therefore, an optical deflectoraccording to the invention can very accurately control the operation ofthe deflection means in such a way that it is not affected by changes ofenvironmental temperature of the deflection means and the detectioncircuit.

1. An optical deflector having deflection means for deflecting modulatedlight from a light source so as to make deflected beams of light scan,said optical deflector comprising a control means for gauging a distancebetween a position of a deflected beam of light moving on a lightreceiving element in one direction and a position of another deflectedbeam of light moving on the light receiving element in the oppositedirection and controlling at least either said light source or saiddeflection means so as to make the distance agree with a predeterminedvalue.
 2. An optical deflector according to claim 1, further comprisingmeans for detecting a phase difference between a phase of the deflectedbeams of light and a phase predefined in said optical deflector from aposition of the deflected beam of light moving in the one direction anda position of the deflected beam of light moving in the oppositedirection and specifying means for specifying at least either a drivefrequency to be applied to said optical deflector or a modulation timingof forward and backward image drawing on the basis of the phasedifference.
 3. An optical deflector according to claim 1, wherein saidlight receiving element has a plurality of light receiving sections thatare arranged along the one moving direction of the deflected beam oflight.
 4. An optical deflector according to claim 1, wherein said lightreceiving element has a plurality of two-dimensionally arranged lightreceiving sections that are arranged along the one moving direction ofthe deflected beam of light and also along in a direction perpendicularto the one direction.
 5. An image forming apparatus comprising anoptical deflector according to claim 1 and a light source.
 6. An imageforming apparatus according to claim 5, adapted to draw an electrostaticimage on an electrophotography type photosensitive body by means offorwardly and backwardly moving deflected beams of light.
 7. An imageforming apparatus according to claim 5, adapted to draw an image on aprojection surface by means of forwardly and backwardly moving deflectedbeams of light.
 8. A method of controlling an optical deflector adaptedto deflect light from a light source so as to make deflected beams oflight scan, said method comprising: a position detecting step ofdetecting a position of a deflected beam of light moving on a lightreceiving element in one direction and a position of another deflectedbeam of light moving on the light receiving element in the oppositedirection; a step of sensing a phase difference between a phase asdetected in said detecting step and a predefined phase; and a step ofcontrolling either a drive frequency being applied to said opticaldeflector or a modulation timing for reciprocative drawing according toan outcome of said sensing step.
 9. A method according to claim 8,wherein said position detecting step is a step of detecting a modulatedpixel position in a photoelectric conversion element by weighting thecenter coordinates of a plurality of light receiving sections on thebasis of information on the detected quantity of light from each of saidplurality of light receiving sections, and adding the weighted centercoordinates of the light receiving sections.
 10. A method according toclaim 8, wherein said position detecting step is a step of determiningthe light receiving section having the largest quantity of light as amodulated pixel position on the basis of information on the detectedquantity of light from each of said plurality of light receivingsections.
 11. A method according to claim 8, wherein said positiondetecting step is a step of detecting a modulated pixel position bymeans of a plurality of photoelectric conversion elements arrangedtwo-dimensionally and contained in said light receiving element and isalso a step of detecting the positions of the deflected beams of lightdisplaced from each other in a direction perpendicular to the scanningdirection and corresponding respectively to the scanning period in saidone direction and the scanning period in said opposite direction.